YANBU. Incorrect spelling and grammar proliferate more and more. Last week there were simultaneously half a dozen threads on here with spelling mistakes in their titles, never mind their content! It set my teeth on edge.
Having said that, I don't think I've ever pointed out mistakes to the OP because it's not the point of their post (and possibly because somebody else always already has!)
Some people may have entirely justifiable reasons for their lack of literacy but most don't, in my belief; they just can't be bothered to get it right and don't think it matters anyway. Whatever the reason, it is still a lack of literacy and those of us who find it annoying are literally not wrong.